Development apparatus for electrostatic copiers



Dec. 10, 1968 G. zwElG 3,415,223

DEVELOPMENT APPARATUS FOR ELECTROSTATIC COPIERS Filed April 24, 1967 3 Sheets-Sheet 1 44 INVENT'OR. y 7% f 54 700? 64,1116?? Zzuef/g Blair, zzdfles', Cesari@ 2% 5'?. (Inge arrow/MVS,

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DEVELOPMENT APPARATUS FOR ELECTROSTATIC COPIERS 3 Sheets-Sheet 2 Filed April 24, 1967 C m; .V5

gwn@ L IwIIJ G. ZWEIG Dec. 10, 1968 DEVELOPMENT APPARATUS FOR ELECTROSTATIC COPIERS 5 Sheets-Sheet 3 Filed April 24, 1967 INVENTQR. 'er Z we@ United States Patent 3,415,223 DEVELOPMENT APPARATUS FOR ELECTROSTATIC CGPIERS Gilbert Zweig, Stamford, Conn., assignor to Pitney-Bowes, Inc., Stamford, Conn., a corporation of Delaware Filed Apr. 24, 1967, Ser. No. 633,262 21 Claims. (Cl. 118-637) ABSTRACT F THE DISCLOSURE A developer tank contains a liquid developer, consisting of a dispersant having electrical insulating properties in which is dispersed electroscopic carbon black toner particles. Guides mounted on the tank position a coated paper bearing an electrostatic image pattern as it is moved through the liquid developer. A development electrode in the form of parallel, elongated metallic rods is submerged in the liquid developer with the rods extending perpendicular to the direction of paper travel. The rods, in intimate proximity to the paper but insulated therefrom by a thin film of liquid developer, are maintained at a potential different from the electrostatic image pattern, thus producing an electrostatic field therebetween; this field causing the dispersant to circulate toward the individual rods and away from the paper adjacent the region of minimum clearance therebetween, while the particles are drawn into this minimum clearance region. With adjacent rods in lengthwise parallel contact, containment zones are provided for accommodating cataphoretic movement of the dispersant and particles in relation to each rod. From this circulatory movement toner particles are deposited on the paper surface to develop the image pattern.

Background of the invention The problem of edge effect has plagued electrostatic copiers since their inception. This problem arises when large areas of electrostatic charge are ultimately developed such as a large black area on a copy, The lines of force due to the charges of a large electrostatically charged area are more concentrated at the preiphery of the charged area and are not properly oriented in the interiors of such areas to attract sufficient toner particles in -a reasonable time period to adequately develop the central image portion. In the development of such an electrostatic image, the deposition rate of toner particles in dependent upon the orientation of these electrostatic lines of force. Consequently, the resultant reproduction of such large area electrostatic images results in the edges being emphasized as intensely dark with the center of the area being very faded.

To overcome this problem, Various forms and varieties of development electrodes-disposed adjacent the electrostatic image have been proposed. These development electrodes help somewhat to reorient the electrostatic eld lines of force, but have not been effective i materially enhancing the quality of copy with regard to edge effect. These prior art electrodes are generally mechanically spaced from the paper to prevent grounding of the electrostatic charge on the paper. This was generally accomplished by a mesh of insulating material placed between the electrode and the paper. I have found that such insulation is too thick and that a film of liquid developer on the paper maintains optimum spacing between the electrode and the paper and yet provides sufficient electrical insulation so that the image charge on the paper surface is not adversely affected.

Summary of the invention My invention relates to development apparatus for use in electrostatic copies of the type employing a liquid lCC developer of toner particles dispersed in a liquid dispersant. I employ a development electrode which is disposed in intimate proximity with a surface bearing an electrostatically charged pattern conforming to the image to be copied. The surface is pre-immersed in the liquid developer and then drawn past the surface of the electrode; the liquid developer being in the form of a thin film over the paper or other surface to insulate it from the electrode and prevent discharge of the electrostatic image. The electrode causes the lines of force to lbecome uniform over large black areas of the electrostatically reproduced image, preventing undue concentration of force lines at the periphery of these large areas. The development electrode further presents a surface configuration to the electrostatic image which provides regions of minimum clearance separated by regions of greater clearance called containment Zones. The development electrode in intimate physical relationship to the electrostatic image provides an electrostatic field with lines of force extending directly from the image to the electrode. The surface configuration of my electrode is effective to cause cataphoretic movement of the dispersant and toner particles in circulatory paths in the containment zones, whereupon toner particles are deposited on the large area electrostatic image at increased rates.

Thus my invention provides for acceleration of the development process in electrostatic copiers, as well as effectively and efficiently overcoming the problem of edge effect. In addition, wide variations of toner concentration in the dispersant are tolerable in making acceptable copies. The apparatus of my invention is inexpensive to manufacture, simple in design, and efficient in operation. Electrostatic copiers equipped with my invention become more versatile in their copying capabilities, being able to faithfully copy photographs, blueprints, photostats, and other like subject matter having large, dark areas to be duplicated. In addition, the development process may be speeded up, thus reducing the time required to make each copy.

My invention comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

Description of the drawings FIGURE l is a simplified side elevational View of an electrostatic copier employing my invention.

FIGURE 2 is a top plan view of a developer tank and development electrode .of my invention.

FIGURE 3 is a sectional view taken along lines 3-3 of FIGURE 2.

FIGURE 4 is a sectional view taken along line 4-4 of FIGURE 2.

FIGURE 5 is an exaggerated diagrammatic illustration showing the manner in which the development electrode accelerates the disposition of toner particles on an electrostatic image.

Description of the preferred embodiment of the invention The development apparatus of my invention is particularly useful in electrostatic copiers of the type generally shown in FIGURE l. An original document 10 to be copied is inserted face up in a slot 11. The original 10, as it passes through slot 11, is inverted yby a chute 12 and then conveyed by pinch rollers 13 to an exposure station generally indicated at 14. A light source 16 scans the document 10, and the reflected image is directed by a mirror 18 through a lens assembly 20 for projection at an image station generally indicated at 22.

Copy paper having a suitable photoconductive coating, such as Electrofax paper having a zinc oxide coating on one side, is drawn from a supply roll 24 by pinch rollers 25. This paper is preferaby cut into sheet length corresponding to the length of the original by a cutter mechanism generally indicated at 27. The sheet is then conveyed by pinch rollers 28 to a corona charger, generally indicated at 30, operating to place a uniform electrostatic charge on the photoconductive coating. The sheet is then conveyed by pinch rollers 31 so as to be positioned at the image station 22 just in advance of the time when the light source I6 begins scanning the original It) positioned at the exposure station 14.

In accordance with well-known electrostatic copying techniques, the image to be copied is then projected by the lens system on the photoconductive layer in contacting engagement with a conductive substrato of the sheet; the sheet being indicated at 26 as positioned at the image station 22. The conductive substrate is effective to selectively discharge the uniform electrostatic charge on the photoconductive layer in correspondence with the image light intensity projected by lens system 20. Thus the optical image to be copied is transformed into a latent electrostatic image on the zinc oxide layer of the sheet 26. The sheet is then conveyed Vby pinch rollers 34 to a development station, generally indicated at 36, where the electrostatic image carried thereby is developed, i.e., rendered visible. From the development station 36, the developed sheet 26 is conveyed by pinch rollers 37 to a drying station, generally indicated at 38 and then to a compartment, generally indicated at 39, where copies are accumulated. A hopper 39a catches the original 10 as it is withdrawn from the exposure station 14 by pinch rolers 14a.

The development station 36, seen in greater detail in FIGURES 2 through 4, includes a developer tank 40 holding a quantity of liquid developer consisting of electroscopic toner particles dispersed in a liquid dispersant. The tank 40 may be a molded plastic unit provided with a molded plastic top 42 supported on and affixed to the upper edge of the tank by a plurality of screws 43. A series of posts 44 upstanding from the bottom of tank 40 are provided for additional support of the top 42.

The top 42 is provided with an upstanding mounting sleeve 45 surrounding an aperture 46 which receives an inverted canister 48 containing a supply of liquid developer (FIGURE 3). The downwardly facing dispensing end of the canister 48 is provided with a spring biased valve 49, the stem thereof being disposed in engagement with the top of the pedestal 50 extending upwardly from the bottom of tank 4t). As seen in FIGURE l, an actuator mechanism, generally indicated at 52, is automatically or manually operated to push the canister 48 downwardly and open valve 49 to release the liquid developer into the tank 40. Actuator mechanism 52 need be operated only periodically in order to maintain the developer level in tank 40 within desired limits.

To guide the sheets 26 bearing the electrostatic image through the dispersant in tank 40, the top 42 is provided with a plurality of arcuate lower rib-like guides 54 distributed along the length of the tank 40 as seen in FIG- URES 2 and 4. It is to be noted that the lower guides 54 are positioned such as to provide significant clearance between them and the bottom of the tank 40. A molded plastic plate 56 supported by the top and afxed in place by screws 57 (FIGURES 2 and 4) carries a plurality of spaced, substantially semi-circular vane-like upper guides 58 depending from its under surface and each being in closely spaced vertical relationship to one of the lower guides 54. Thus, the guides 54 and 58 define an arcuate passageway 60 (FIGURE 3) effective to guide the sheets 26 along a predetermined path into, through, and out of the developer bath in tank 40. As seen in FIGURES 2 and 4, the guides 54 and 58 on the right and left of center are skewed outwardly so as to prevent the sheets 26 from wrinkling as they travel through passageway 60.

The individual upper guides 58 are notched as indicated at 58a (FIGURE 3) to accommodate a development electrode generally indicated at 70, extending substantially thc length of tank 40 as seen in FIGURES 2 and 4. The development electrode 'itt consists of a plurality of electrically conductive rods Nia-d formed of a suitable metal such as aluminum. As will be appreciated from the description to follow, at least two rods are required and four has proven to provide good results in eliminating edge effect. The development electrode rods 70r1-70zl, while being accommodated in the notch 58a of guides 58. are wholly supported along their lengths by the lower guides 54. Thus, sheets 26 in passing through passageway 60 are moved under the electrode rods with the zinc oxide layer in intimate proximity therewith.

An agitator, generally indicated at St) in FIGURES 2 and 4. is mounted on a vertical shaft journalled adjacent its ends by the top 42 and the bottom of tank 40. A coupling 8l fixed on the upper end of the agitator shaft above top 42 is adapted for driving engagement with an electrical motor (not shown). Rotation of the agitator effects circulation of the developer in tank 40.

For detailed consideration of the operation of the development electrode 70, reference is made to FIGURE 5 wherein the physical relationship of the electrode rods 70a-70d and zinc oxide coated sheet 26 is shown in exaggeration. It `will be appreciated that `while the electrode rods are shown lin spaced relationship with each other and with the paper sheet 26, in actuality, they are in intimate proximity with each other, being spaced apart several microinches by an intermediate film of liquid developer. Thus it is an important feature of the invention that the sheet 26 be pre-immersed in liquid developer or dispersant alone prior to being moved under the electrode rods.

The electrode rods 7011-701! are electrically connected by spring contacts (FIGURE 4) bearing against the ends of the eelctrode rods; these spring contacts in turn being electrically connected to ground as indicated diagrammatically at 90a. The spring contacts 90 are mounted by ribs 92 depending from plate 56 adjacent each end of the electrode rods 7061-7051 (FIGURES 2 and 4). The spring contacts 90 additionally serve to constrain axial movement of the electrode rods but allow for individual rotation thereof while preserving the ground connection.

The electrodes rods Nia-70d are electrically connected that of the electrostatically charged image, typically negative, borne by the zinc oxide coating layer, indicated at 26a, of the sheet 26. The zinc oxide layer 26u is supported on a relatively conductive paper substrate 2611. An electrically conductive screen 94 (FIGURES 2, 4 and 5) is secured to the bottom edges of guides 54 and extends substantially co-extensively with the length and width of the development electrode 70. The screen 94 is electrically connected to ground, as indicated diagrammatically at 94a, to establish aground potential reference plane for the electrostatic field between the charged image and thc electrode rods.

As previously noted, the thin film of liquid developer between the electrode rods and the paper 26 prevents discharge of the electrostatic image borne by zinc oxide layer 26a. The electrostatic lines of force resulting from the charges included 4in that portion of the image beneath the electrode rods are oriented to extend directly to the electrode rods with minimum fringing. This is one factor contributing to the solution of the edge effect problem by the disclosed apparatus. It is to be noted that by virtue of the intimate physical relationship between the image and the development electrode achieved herein, the configuration of the electrostatic field is much improved over that achieved by prior art apparatus.

Also significant, however, is the contribution of the geometry of the development electrode 70 with its plural, side-by-side electrode rods 70a-70d in overcoming edge effect. As seen in FIGURE 5, there are a plurality of maximum field intensity regions, indicated at 10() where the clearance between the individual electrode rods and the electrostatic image is a minimum. Between these regions 100 are regions of greater clearance constituting containment zones 102 bounded by the paper 26 and the surfaces of the adjacent electrode rods. An illustrative range for the radius of curvature of the individual rod surfaces confronting the image would be in the order of .020 inch to .500 inch; the preferable rage being from .040 to .250 inch. A series of .125 inch diameter rods have given good results here.

If it is assumed that the electrostatic image is selectively negatively charged according to the image to be reproduced, the development electrode rods, being grounded, are at a relative positive potential. When an electrostatic field is between the copy sheet and the electrode a depletion zone 104 develops around the relatively positive electrode rods from which the toner particles, having a net positive charge, are repelled, leaving clear dispersant having a net negative charge. Toner particles are generally drawn into the region 100 of maximum rfield intensity and are attracted downwardly toward the charged image. Thereafter, the lines of force resulting from the electrostatic interaction between the selectively charged portions on the photoconductive layer of sheet 26 and the adjacent portions of the electrode 70 tend to generate a cataphoretic circuation of toner particles and dispersant, such circulation being very generally illustrated by the fiow lines 105 of FIGURE 5, this cataphoretic circulation greatly facilitates a more copious deposition of toner particles on the charged copy sheet.

It is understood that while this circulatory movement of the toner particles and dispersant is taking place, the paper is being continuously fed through the development station 36, and thus the development of the electrostatic image is performed in stages as it moves under the electrode rods 70r1-70d and successively past containment zones 102. The containment zones 102 provide environments, free from perturbations in the developer bath, for the above-described circulatory movement of the dispersant and toner :in close proximity to each electrode rod 70a-70d. Thus, greater concentrations of toner particles are deposited on the charged image in less time. The development process is accelerated and large area images are filled in substantially uniformly with the resulting elimination of edge effect.

It will be noted that the electrode rods 70er-70d are free to rotate as the paper feeds by. This serves to introduce into the containment zones 102 developer from the bath proper, thus automatically replenishing the toner concentration in the containment zones. The grounding of the electrode rods prevents them from accumulating a negative charge from contiguous negatively charged images. This, in turn, prevents toner particles from plating out on the electrode rods and being offset onto the paper 26 one rod revolution later, producing ghosts The tendency for ghosting is also reduced by the presence of the grounded screen 94.

In a practical embodiment of my invention, the electrode rods ma-'70d were aluminum, 5%; inch in diameter and ll inches long. The toner consisted of carbon black particles coated lwith a suitable resin to enhance adhesion to the paper 26. The dispersant used `was Isopar-G, made by the Hulmble Oil & Refining Company. Other types of dispersants may be employed as long as their viscosity and electrical insulating characteristics are such as to produce the requisite insulating film between the electrode rods and the charged image.

It will be appreciated that the development electrode 70 may take other forms, although the plural rods 70a- 70b are preferred. For example, a reasonably satisfactory development electrode pursuant to my invention may be formed as a single unit having an overall surface configuration calculated to provide in combination with the paper 26 at least one minimum clearance region 100 fianked by at least one containment zone 102. In fact, the described circulatory movement of dispersant and toner has been achieved by drawing a paper sheet bearing a charged image past the rounded tip of a single development electrode extending above the dispersant level. If the sheet is pre-immersed in the developer bath prior to being drawn past the electrode, a containment Zone is created by a meniscus of developer developing between the sheet and the trailing surface of the electrode tip.

If desired, rods 70a-70d may be rotatable driven by external drive means to assist in feeding the `copy sheet through the developer tank.

Since certain lchanges may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In an electrostatic copier, development apparatus comprising, in combination:

A. a container containing a dispersion of a fiuid and electroscopic particles (l) said container adapted to receive a sheet bearing surface electrostatic charges in image formation,

(a) said sheet moving in a path leading into and through said dispersion; and B. a development electrode immersed in said dispersion in close proximity to and above said sheet as it is moved,

(l) said electrode establishing an electrostatic field between it and said electrostatic image, and (2) having a surface configuration submerged in said dispersion contiguous to said sheet path for providing two regions of minimum clearance between it and said electrostatic image separated by a region of greater clearance therebetween,

(a) said region of the greater clearance constituting in combination with the sheet and said minimum clearance regions a containment zone separate from said dispersion proper wherein the fluid and particles move in a circulatory pattern under the infiuence of said electrostatic field, whereby the distribution of the deposition of said particles on said electrostatic image is improved.

2. The apparatus defined in claim 1 wherein the surface configuration of said development electrode is such as to provide a plurality of minimum clearance regions, and intermediate greater clearance regions.

3. The apparatus defined in claim 2 wherein said minimum and greater clearance regions are spaced from one -another in the direction of movement of the sheet.

4. The apparatus defined in claim 1 wherein said electrode surface is (l) curvilinear in the direction of movement of the sheet, and

(2) disposed in tangential relationship to said path.

5. The apparatus defined in claim 4 wherein (l) said electrode surface providing said minimum Clearance region and portions of adjacent greater clearance regions has a radius of curvature in the range of 0.40 to .250 inches.

6. The apparatus defined in claim 1 wherein (l)said development electrode is electrically connected to ground.

7. The apparatus defined in claim 6 which further includes C. an electrically conductive screen disposed closely adjacent the opposite side of said path from said electrode, said screen (1) extending coextensively with said electrode and electrically connected to ground.

8. The apparatus defined in claim 1 wherein (1) said development electrode consist of at least two elongated, electrically conductive rods disposed in close proximity with each other along their lengths, said rods (a) extending transversely to the direction of movement of the sheet, and

(b) lying in trangetial relation to said path,

(c) said minimum clearance region being centered about the point of tangential relationship between the sheet and each said rod, and

(d) said containment Zone being bounded by the surfaces of adjacent rods and the sheet between said minimum clearance regions.

9. The apparatus defined in claim 8 which further includes C. Spring contacts physically engaging and electrically contacting at least one end of each said rod (1) each said spring contacts being electrically connected to ground.

10. The apparatus defined in claim 8 wherein (l) said rods are supported on the sheet in contiguous relation to the electrostatic image and electrically insulated therefrom lby a thin film of said fluid.

11. The apparatus defined in claim 10 wherein (1) said rods are rotatably driven about their lengthwise axis,

(2) whereby to introduce additional particles into said containment zone.

12. The apparatus dened in claim 8 wherein (l) said container mounts a plurality of spaced lower guides and a plurality of correspondingly spaced upper guides,

(a) the corresponding upper and lower guides being spaced to define said path,

(2) means forming a notch in each said upper guide for accommodating said rods,

(3) said rods being supported on said lower guides in the absence of the sheet.

13. In an electrostatic copier, apparatus for developing an electrostatically charged image borne by the surface of a sheet, said apparatus comprising, in combination:

A. a bath containing a dispersion of pigment particles in a uid dispersant,

(1) means for pre-immersing the surface in said dispersion to form an electrically insulating film thereon.

B. a development electrode above the image;

(1) means for bringing said electrode and the submerged image into intimate mutual contact but electrically insulated from each other by said film,

(2) said electrode and the image being of different potentials to produce an electrostatic field therebetween,

(3) said electrode having a surface configuration relative to the sheet surface such as to provide at least two regions of minimum clearance therebetween separated by a region of greater clearance forming in combination with the sheet a containment zone.

14. The apparatus defined in claim 13 which further includes C. means for producing relative movement between said electrode and the sheet.

15. In an electrostatic type copier having a copy sheet developing station that includes a tank containing a liquid developer, said liquid developer comprising a dispersant with electroscopic particles therein;

A. means for guiding a selectively charged copy sheet along a predetermined path through said liquid developer;

B. containment means for establishing at least one localized containment zone in said liquid developer,

(l) said Zone extending above and across the effective width of said predetermined copy sheet feed path and serving n conjunction with the copy sheet to effectively isolate a small portion of said liquid developer from the remaining portions of said liquid developer; and

C. means for electrostatically conditioning said containment means so as to establish an electrostatic field between a charged copy sheet and said containment means so as to establish a cataphoretic circulation of liquid developer' in said containment Zone,

(l) said cataphoretic circulation serving to produce a more uniform distribution of deposition of said electroscopic particles on said copy sheet.

16. Apparatus as dened by claim 15 wherein that portion of said containment means adjacent the said feed path is formed of two tangentially positioned surfaces at least one of which is curved and has an effective radius of curvature in the range of from 0.20 to .500 inch.

17. Apparatus as defined by claim 16 wherein said radius of curvature is in the order of .125 inch.

18. Apparatus as defined by claim 15 wherein said containment means comprises at least one metallic rod that is disposed in said liquid developer adjacent said predetermined copy sheet feed path.

19. Apparatus as defined by claim 15 wherein said containment means comprises a plurality of relatively thin, closely spaced metallic rods disposed in said liquid developer adjacent said predetermined feed path.

20. Apparatus as defined in claim 15 wherein said containment means includes a plurality of rods that are loosely mounted in said developer tank both for rotation about their longitudinal axes and for at least a limited amount of translational movement in directions normal to their respective longitudinal axes.

21. Apparatus as dened by claim 18 wherein said rods are connected to an electrostatic potential that is more positive than that of the said charged copy sheet.

References Cited UNITED STATES PATENTS l 3,169,887 2/1965 York 118-637 3,203,395 8/1965 Liller 118-637 3,236,168 2/1966 Rautbord et al. 95-89 3,245,330 4/1966 Okishma 95-1.7 3,249,088 5/ 1966 Ostensen 118-637 3,256,855 6/1966 Oliphant 118-637 3,263,648 8/1966 Chapman et al 11S-49.1 3,299,787 1/1967 Kolb et al. 95-1.7

FOREIGN PATENTS 938,349 10/1963 Great Britain.

PETER FELDMAN, Primary Examiner.

U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,415 223 December 10 1968 Gilbert Zweig It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 7, "retable" should read ratatably line 66 "O .40" Should read 040 Signed and Sealed this 19th day of May 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

